1. Field of the Invention
The present invention relates generally to the field of geophysical electrical measurements. More particularly, it concerns methods and apparatus for measuring an electrical property of the ground by utilizing a polarizable electrode for transmitting a current into the ground and a non-polarizable electrode for measuring a potential associated with that current.
2. Description of Related Art
Various geophysical applications utilize electrical surveys to determine a sub-ground resistivity distribution by making electrical measurements on the ground. From such measurements, the resistivity of the sub-ground may be estimated and related to various geological parameters such as mineral and fluid content, porosity, and water saturation.
Resistivity measurements are typically made by applying current directly into the ground using a pair of transmitting electrodes. Resulting potential differences may then be measured directly using several other receiving electrodes. The receiving electrodes are typically arranged in an array or grid.
Induced polarization relies upon the fact that current injected into the ground causes some materials to become polarized. The phenomenon of polarizing the ground is often referred to as chargeability. The type and amount of chargeability associated with the ground may be measured by taking time domain and/or frequency domain measurements. In general, such measurements determine the degree to which the ground has been polarized by comparing a source signal transmitted to the ground with a measured decay signal after the source signal has been shut off. One or more physical properties of the ground may be correlated with the degree of polarization.
There are two main types of induced polarization systems in use today. One is an automated system that has a distributed cable with multiple configurable electrodes on it that may be configured as transmitting or receiving electrodes. Each electrode typically contacts the ground with a single, metal-stake contact. In these systems, both the transmitting electrodes and the receiving electrodes use the same metal-stake contact. With regard to transmission of current, the metal-stake contacts are advantageous in that they exhibit low impedance with the ground and thus may provide a high level of injected current. Although advantageous in this regard, the receiving electrodes, however, are often hindered by the use of the metal-stake contact when measuring the decaying potential after the transmitted current is shut off. Without being bound by theory, it is believed that the use of the metal-stake contact may create a self potential between the stake and the ground. This self potential may interfere with the induced polarization level in the ground, which may negatively affect the accuracy of data being gathered. For at least this reason, it would be advantageous to provide the ability to measure the electrical potential of the ground using a contact member that does not create a self potential between itself and the ground.
Another type of induced polarization system currently utilizes a manual method a that employs a set jig having multiple receiving electrodes on it that are made of either non-polarizing or polarizing material. Although systems of this type have shown some degree of utility for measuring electrical properties of the ground, they suffer disadvantages including the fact that they are often tedious to set up and measurements often take a long time. Therefore, it would be advantageous to provide for the ability to measure an electrical property of the ground using an automated system which is easy to set up and which may make measurements in a short period of time.
In one respect, the invention is an apparatus for measuring an electrical property of a ground. As used herein, the indefinite articles xe2x80x9caxe2x80x9d and xe2x80x9canxe2x80x9d are to connote xe2x80x9cone or morexe2x80x9d unless otherwise noted. As used herein, by xe2x80x9cground,xe2x80x9d it is meant any surface or sub-surface for which an electrical property may be measured. xe2x80x9cGroundxe2x80x9d may refer to, but is not limited to, the surface of the earth, surface of lakes, rivers, oceans, bottoms of lakes, rivers, oceans, holes, and/or any man-made structures. The apparatus includes a first contact member, an electrode, and a second contact member. The first contact member is configured to transmit a current to the ground. The electrode includes a first conductor and a second conductor. The first conductor is coupled to the first contact member and is configured to transmit the current through the contact member to the ground. The second contact member is coupled to the second conductor and is configured to measure an electrical potential associated with the current. As used herein, by xe2x80x9celectrical potential,xe2x80x9d it is meant any potential. For example, xe2x80x9celectrical potentialxe2x80x9d applies to, but is not limited to, potentials associated with, for instance, resistivity and/or induced polarization.
In other respects, the electrical property may include resistivity. The electrical property may include induced polarization. The first contact member may be polarizable and the second contact member may be non-polarizable. The first contact member may include a metal stake and the second contact member may include a non-polarizable electrode. The first contact member may include a metal stake. The metal stake may include a conducting ledge configured to secure the electrode. The apparatus may also include a resistivity meter coupled to the electrode and configured to record the electrical potential. The apparatus may also include an interface box coupled between the resistivity meter and the electrode. The electrode may be within a Wenner alpha array, a Wenner beta array, a Wenner gamma array, a pole-pole array, a dipole-dipole array, a pole-dipole array, a Wenner array, a Schlumberger array, an equatorial dipole-dipole array, or any combination thereof.
In another respect, the invention is a system for measuring induced polarization of a ground. The system includes a polarizable contact member, an electrode, a non-polarizable contact member, and a resistivity meter. The polarizable contact member is configured to transmit a current to the ground. The electrode includes a first conductor and a second conductor. The first conductor is coupled to the polarizable contact member and is configured to transmit the current through the polarizable contact member to the ground. The non-polarizable contact member is coupled to the second conductor and is configured to measure an electrical potential associated with the current. The resistivity meter is coupled to the electrode and is configured to record the electrical potential.
In other respects, the system may also include an interface box coupled between the resistivity meter and the electrode. The system may also include software configured to render a map of the ground from the electrical potential recorded by the resistivity meter. The electrode may be within a Wenner alpha array, a Wenner beta array, a Wenner gamma array, a pole-pole array, a dipole-dipole array, a pole-dipole array, a Wenner array, a Schlumberger array, an equatorial dipole-dipole array, or any combination thereof.
In another respect, the invention is a method for measuring an electrical property of a ground. The method includes delivering a current to the ground through an electrode and a polarizable contact member, the electrode including a first and a second conductor, and the first conductor being coupled to the polarizable contact member. An electrical potential associated with the current is measured with a non-polarizable contact member coupled to the second conductor, and the electrical potential is recorded with a resistivity meter coupled to the electrode.
In other respects, the electrical property may include resistivity. The electrical property may include induced polarization. The electrode may be within an array, and wherein the measuring comprises a Wenner alpha measurement, a Wenner beta measurement, a Wenner gamma measurement, a pole-pole measurement, a dipole-dipole measurement, a pole-dipole measurement, a Wenner measurement, a Schliumberger measurement, an equatorial dipole-dipole measurement, or any combination thereof. The method may also include rendering a map of the ground from the electrical potential recorded by the resistivity meter.